Many extensions have a need to draw UI over the page. Developers have traditionally relied on the stacking context and specific properties like z-index. These have always been limited but for the most part, do the job.

With the introduction of modals (via the <dialog> element), web developers can now draw to a new "top layer" that renders above everything else. Example: https://codepen.io/oliverdunk/pen/xxWKjOZ. This makes it much harder for extension developers as they now have to ensure UI is inside of the highest modal, and this could involve repeatedly moving their UI between DOM elements.

The ideal solution here would be a standardized mechanism - maybe a new layer - that extensions could put UI on. This would be reliable even as new standards are developed and provide the most robust long-term solution.